1. Field of the Invention
This invention relates to a cascade hydroisomerization-hydrodewaxing process, and particularly to a dual catalyst cascade hydroisomerization-hydrodewaxing process utilizing a medium pore crystalline zeolite catalyst in one reaction zone and a different medium pore crystalline zeolite catalyst in another reaction zone.
2. Discussion of Prior Art
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which may function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require a considerable resort to empiricism in practical refining. Underlying these quantitative uncertanties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 450.degree. F. (232.degree. C.), the molecular weights of the hydrocarbon constituents are high and these constituents display almost all conceivable structure types. This complexity and its consequences are referred to in well-known treatises, such as, for example, "Petroleum Refinery Engineering", by W. L. Nelson, McGraw-Hill Book Company, Inc., New York, NY, 1958 (Fourth Edition). For purposes of this invention, lubricating oil or lube oil is that part of a hydrocarbon feedstock having a boiling point of 650.degree. F. (343.degree. C.) or higher as determined by ASTM D-97 test method.
In general, the basic premise in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set or properties, such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined, one would reconstitute the crude oil.
A refined lubricant stock may be used by itself, or it may be blended with another refined lubricant stock having different properties. Or, the refined lubricant stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, V.I. improvers.
For the preparation of a high grade distillate lubricating oil stock, it is known to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450.degree. F. to 1050.degree. F. (232.degree.-566.degree. C.). After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g., furfural, phenol, sulfolane, or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example, by admixing with a solvent, such as a blend of methylethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes, which are then separated from the raffinate. Sufficient quantities of wax are removed to provide the desired pour point for the raffinate.
Other processes, such as hydrofinishing or clay percolation, may be used if needed to reduce the nitrogen and sulfur content or improve the color of the lubricating oil stock.
Viscosity index (V.I.) is a quality parameter of considerable importance for distillate lubricating oils to be used in automotive engines and aircraft engines subject to wide variations in temperature. This index indicates the degree of change of viscosity with temperature. A high V.I. of 100 indicates an oil that does not tend to become viscous at low temperature or become thin at high temperatures. Measurement of the Saybolt Universal Viscosity of an oil at 100.degree. F. (38.degree. C.) and 210.degree. F. (99.degree.), and referral to correlations, provides a measure of the V.I. of the oil. For purposes of the present invention, whenever V.I. is referred to, it is meant the V.I. as noted in the Viscosity Index tabulations of the ASTM (D567), published by ASTM, 1916 Race Street, Philadelphia, PA, or equivalent.
In recent years, catalytic techniques have become available for dewaxing of petroleum stocks. A process of that nature developed by British Petroleum, in which a Mordenite type of molecular sieve catalyst is used, is described in The Oil and Gas Journal, dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113.
U.S. Pat. No. Re. 28,398 describes a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such a process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938.
U.S. Pat. No. 3,956,102 discloses a particular method for dewaxing a petroleum distillate with a ZSM-5 catalyst. Typical aging curves are shown in Sheet 2 of the drawing of the U.S. Pat. No. 3,956,102.
U.S. Pat. No. 3,769,202 teaches catalytic conversion of hydrocarbons using as a catalyst two different crystalline silicate zeolites, one having a pore size greater than 8 Angstroms and the other having a pore size less than 7 Angstroms. This reference teaches that a conventional hydrogenation/dehydrogenation component may be added, in an amount from about 0.01 to about 30 wt %.
U.S. Pat. No. 3,764,520 teaches hydrocracking with a catalyst mixture of large and small pore crystalline zeolites.
U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point.
U.S. Pat. No. 4,053,532 is directed towards a hydrodewaxing operation involving a Fischer-Tropsch synthesis product utilizing ZSM-5 type zeolites.
U.S. Pat. No. 3,956,102 is connected with a process involving the hydrodewaxing of petroleum distillates utilizing a ZSM-5 type zeolite catalyst.
U.S. Pat. No. 4,247,388 describes dewaxing operations utilizing ZSM-5 type zeolites of specific activity.
U.S. Pat. No. 4,222,855 describes dewaxing operations to produce lubricating oils of low pour point and of high V.I. utilizing zeolites including ZSM-23 and ZSM-35.
U.S. Pat. No. 4,372,839 describes a method for dewaxing crude oils of high wax content by contacting the oils with two different zeolites, e.g., ZSM-5 and ZSM-35.
U.S. Pat. No. 4,419,220 describes a dewaxing process utilizing a Zeolite Beta catalyst.
U.S. Pat. No. 4,383,913 discloses a cascade process wherein the hydrocarbon feed first passes over a bed of zeolite containing catalyst, and then over a bed of amorphous catalyst.
U.S. Pat. No. 3,894,937 discloses a dual catalyst converter and process, suitable for cascade processing.
Copending U.S. patent application Ser. No. 661,632, filed 10/17/84 which is incorporated herein by reference, describes a lubricating oil dewaxing process utilizing a zeolite catalyst having a Constraint Index not less than 2; an acidic catalytic material selected from the group consisting of Mordenite, TEA Modenite, Dealuminized Y, Rare Earth Y, amorphous silica alumina, chlorinated alumina, ZSM-4 and ZSM-20; and a hydrogenation component.
Copending U.S. patent application Ser. No. 631,681, filed 7/16/84 which is incorporated herein by reference, describes a process for dewaxing a hydrocarbon feedstock using a catalyst containing, in combination, a zeolite with a Constraint Index greater than 1, a second, different catalyst component and a hydrogenation component.
Despite all the advances that have been made in catalytic hydrodewaxing, it would still be beneficial if processes could be devised which would provide increased lube yield, higher viscosity index (V.I.), improved catalyst stability and flexibility in catalyst regeneration.
It is an object of the present invention to overcome the deficiencies of the prior art.
It is additionally an object of the present invention to provide an improved process for producing a dewaxed lubricating oil having higher V.I., increased lubricating oil yield, improved catalyst stability and increased flexibility in catalyst regeneration.
These and other objects are fulfilled by the present invention, which is disclosed below.